The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to electromagnetic wave devices and more particularly to polarization selective screens allowing the transmission of only such energy having a predetermined linear polarization.
2. Description of the Prior Art
Polarization selective screens (or grids) are well known in the art. The polarization of a propagating electromagnetic wave is generally considered less important for frequencies below about 50 MHz because at these relatively lower frequencies, the polarization of a wave is often unpredictably distorted by the effects of reflection, diffraction and refraction. On the other hand, at VHF, UHF and especially microwave frequencies, transmissions between two points is usually line-of-sight where less reflection, refraction or diffraction effects are generally experienced.
At UHF and microwave frequencies it has been found to be advantageous in some applications to differentiate between different polarizations. For example, where two incoming signals are to be separately received, but there is not a great separation in frequency between such signals, both signals would cause desensitization of the receiver front end adapted to receive the other signal. However, if these two signals are transmitted with orthogonal polarizations and a polarization selective screen is placed in front of the receiving antenna, a signal, even though relatively close in frequency but having a polarization orthogonal to that of the screen, will be rejected and will not cause the undesired desensitization. In other words, polarization selective screens can be used as highly advantageous filters for separating RF signals which are very closely spaced in frequency, whereas microwave filters capable of such a result are very costly and heavy and usually cannot provide good selectivity of each signal.
One such polarization selective screen, known in the art as a "Venetian blind" type, consists of aluminum sheets sandwiched between plastic foam and encapsulated by fiberglass facesheets. This design has the disadvantage of being heavy, has high insertion loss and is highly susceptible to solar torque in satellite applications.